The present invention generally relates to fabrication of semiconductor devices and more particularly to a chemical amplification resist used for patterning a semiconductor layer, as well as to a fabrication process of semiconductor devices that uses such a chemical amplification resist.
Photolithography is an essential process in the fabrication of semiconductor devices. In the art of photolithography, a resist pattern is formed on a semiconductor substrate or a semiconductor layer by applying thereon a photoresist, followed by exposure and development processes of the resist. After the exposure and development, the semiconductor layer is further subjected to an etching process while using the resist pattern thus obtained as a mask. Thus, the photoresist is required to have sufficient sensitivity against the radiation used in the exposure process and simultaneously a sufficient resistance against the etching.
In the conventional fabrication process of semiconductor devices, the radiation of the g-line having a wavelength of 436 nm, or the radiation of the i-line having a wavelength of 365 nm, has been used in the exposure. On the other hand, the demand for increased integration density of recent integrated circuits now requires a submicron resolution for the exposure process. In order to meet the demand of such a high resolution exposure, the use of deep-ultraviolet radiation having a wavelength of 200-300 nm, particularly the use of radiation from an excimer laser, is studied.
When a conventional, so-called Novolac resist, which is based upon a phenolic resin, is used for an exposure process under such deep-ultraviolet radiation, however, there arises a problem in that the aromatic ring of the photoresist, which increases the resistance of the photoresist against etching, causes a strong optical absorption. As a result of such a strong optical absorption, the conventional Novolac resist cannot achieve the desired resolution.
In view of the foregoing unsatisfactory result of the conventional photoresist, there is a proposal, as disclosed in the Japanese Laid-open Patent Publication 4-39665, wherein the reference proposes a chemical amplification resist of a polymer or a co-polymer of acrylic ester or alpha-substitute acrylate that contains adamantyl group. Hereinafter, the photoresist of the foregoing prior art reference will be reviewed briefly.
The photoresist of the prior art is formed of a co-polymer of adamantyl methacrylate having a structural formula of ##STR1## and tert-butylmethacrylate, and further includes a photoacid generator of triphenylsulfonium hexafluoroantimonate represented by the structural formula of ##STR2##
The photoresist itself is hydrophobic in nature and does not dissolve into alkaline solvent, while when the resist is exposed to short wavelength optical radiation, the foregoing photoacid generator releases a strong Bronsted acid. The Bronsted acid thus released catalyzes the cleavage of polymer pendant groups, acid labile groups capable of regenerating the proton, under appropriate conditions represented as ##STR3##
As a result of such a reaction, further release of the Bronsted acid occurs, and the Bronsted acid thus released further promotes the reaction. As a result of such a reaction, the resist changes to methacrylic acid that is soluble to the alkaline solvent. In other words, one can develop the photoresist by means of the alkaline solvent after exposure.
In the conventional chemical amplification resist described above, it should be noted that one cannot provide the resistance against etching that is comparable to the resistance achieved by the conventional novolac resin, unless adamantyl methacrylate is introduced with a proportion of more than 40 mole %, preferably more than 50 mole % with respect to the tert-butylmethacrylate, as indicated in FIG. 1. In FIG. 1, it should be noted that the etch rate of the photoresist is represented as a function of the composition of the resist wherein the compositional parameters n and m correspond to the parameters n and m of the foregoing structural formula.
On the other hand, such an increase in the proportion of the adamantyl group in the photoresist tends to enhance the hydrophobic nature of the resist, and the wetting of the resist by a polar solvent is degraded as a result. More specifically, the permeation of the alkaline solvent into the photoresist is suppressed with increased proportion of adamantyl group and there occurs a case in which the resist remains unremoved after development in correspondence to the area that has been exposed to the optical radiation. Thus, the conventional chemical amplification resist of the foregoing prior art has a problem of reduced sensitivity when a high resistance against dry etching process is to be maintained.